1. Field of the Invention
The present invention relates to a high-frequency filter, and particularly, to a high-frequency filter which is a plane type distributed constant filter using a dielectric substrate and having plural resonators and is used as, for example, a band-pass filter or the like.
2. Description of the Prior Art
FIG. 18 is a plan view showing an example of a conventional high-frequency filter which is in the background of the present invention, FIG. 19 is a side view thereof. The high-frequency filter 1 includes a dielectric substrate 2 having, for example, a dielectric constant of 10-20. On one whole main face of the dielectric substrate 2, an earth electrode 3 is formed. Also, on the other main face of the dielectric substrate 2, two linear pattern electrodes 4a and 4b are formed so as to be opposite to the earth electrode 3. In this case, the pattern electrodes 4a and 4b are formed in parallel at a fixed interval S1 so as to electromagnetically couple. One end of each of the pattern electrodes 4a and 4b is connected to the earth electrode 3 via one side face of the dielectric substrate 2. One resonator of length .lambda./4 is constructed with the dielectric substrate 2, the earth electrode 3 and one pattern electrode 4a, and another resonator of length .lambda./4 is constructed with the dielectric substrate 2, the earth electrode 3 and the other pattern electrode 4b. Also, input-output electrodes 5a and 5b are formed so as to extend from intermediate parts of the pattern electrodes 4a and 4b to both side parts of the dielectric substrate 2. Thus, the high-frequency filter 1 is constructed as a comb line filter.
FIG. 20 is a plan view showing another example of a conventional high-frequency filter which is in the background of the present invention. In the conventional example shown in FIG. 20, compared with the conventional example shown in FIG. 18 and FIG. 19, the opposite ends of the pattern electrodes 4a and 4b are connected to the earth electrode 3. Thus, the high-frequency filter 1 shown in FIG. 20 is constructed as an interdigital filter.
Also, for example, Japanese publication No. 28441/1985 discloses an example of a conventional strip line filter which is in the background of the present invention. FIG. 21 is a plan view showing an example of such a conventional high-frequency filter, FIG. 22 is a side view thereof. The high-frequency filter 1 shown in FIG. 21 and FIG. 22 includes a dielectric substrate 2 having, for example, a dielectric constant of 10-20. On one whole main face of the dielectric substrate 2, an earth electrode 3 is formed. Also, on the other main face of the dielectric substrate 2, five linear pattern electrodes 4a, 4b, 4c, 4d and 4e are formed so as to be opposite to the earth electrode 3. In this case, the pattern electrodes 4a-4e are formed in parallel at a fixed interval S1 so as to electromagnetically couple each two adjoining pattern electrodes. One end of each of the pattern electrodes 4a-4e is connected to the earth electrode 3 via one side face of the dielectric substrate 2. Thus, five resonators of length .lambda./4 are constructed with the dielectric substrate 2, the earth electrode 3 and the five pattern electrodes 4a-4e. Also, input-output electrodes 5a and 5b are formed so as to extend from intermediate parts of the pattern electrodes 4a and 4e to both sides of the dielectric substrate 2. Thus, the high-frequency filter 1 is constructed as a comb line filter.
In the high-frequency filter 1 shown in FIG. 18 through FIG. 20, for miniaturizing the filter as required in recent years, when the dielectric constant of the dielectric substrate 2 is increased for shortening each length L1 of the pattern electrodes 4a and 4b of the two resonators and so on, an interference effect in an electromagnetic field between the two resonators is too strong. That is, when the dielectric constant of the dielectric substrate 2 is increased, the electromagnetic coupling between the pattern electrodes 4a and 4b of the two resonators is too strong. Thus, frequency characteristics such a selectivity characteristic of the high-frequency filter are deteriorated. For correcting the deterioration of the frequency characteristics, it is necessary to redesign it for extending the interval S1 between the pattern electrodes 4a and 4b of the two resonators and so on. However, when the interval S1 between the pattern electrodes 4a and 4b of the two resonators is extended, the high-frequency filter becomes large, not miniaturized.
Also, in the high-frequency filter 1 shown in FIG. 18 through FIG. 20, since its input-output impedance and an external circuit are matched by drawing the input-output electrodes 5a and 5b from the intermediate parts of the pattern electrodes 4a and 4b directly, the coupling degree between the two resonators is changed by an interference effect in an electromagnetic field, so it is necessary to adjust each distance D1 from the short ends (the ends connected to the earth electrode 3) of the pattern electrodes 4a and 4b to the input-output electrodes 5a and 5b, or each width T of the input-output electrodes 5a and 5b, which makes such a filter complex and difficult to design.
Furthermore, in the high-frequency filter 1 shown in FIG. 18 through FIG. 20, there is a problem that even if the input-output impedance is matched as above mentioned, when the impedance of the external circuit is changed, the frequency characteristic is easily changed too.
Also, in the high-frequency filter 1 shown in FIG. 21 and FIG. 22, for obtaining a high efficiency frequency characteristic having a large attenuation, it is a problem that not only must the resonators be coupled to form a multi-stage filter, which makes the filter large, but also it becomes difficult, involving an exponential function, to design in order to increase the stage number of the resonators.
Furthermore, in the high-frequency filter 1 shown in FIG. 21 and FIG. 22, for miniaturizing the filter as the market has required in recent years, when the dielectric constant of the dielectric substrate 2 is increased for shortening each length L1 of the pattern electrodes 4a-4e of the five resonators and so on, an interference effect in an electromagnetic filed between each two adjoining resonators is too strong. Thus, frequency characteristics such as a selectivity characteristic of the high-frequency filter are deteriorated. For correcting the deterioration of the frequency characteristics, a redesign is necessary for extending the interval S1 between the pattern electrodes of each two adjoining resonators and so on. Then, when the interval S1 between the pattern electrodes of two adjoining resonators is extended, the high-frequency filter becomes large, which interferes with miniaturizing, and it becomes difficult to design it. In some cases, it is impossible to design it.